The present invention relates to a coated cutting tool for chip forming machining.
The Chemical Vapor Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al.sub.2 O.sub.3 as well as of other refractory materials have been discussed extensively in the literature.
The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, with the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al.sub.2 O.sub.3.
Cemented carbide cutting tools coated with various types Al.sub.2 O.sub.3 -coatings, e.g., pure .kappa.-Al.sub.2 O.sub.3, mixtures of .kappa.- and .alpha.-Al.sub.2 O.sub.3 and very coarse-grained .alpha.-Al.sub.2 O.sub.3 have been commercially available for many years. None of these oxide coated products have shown desirable cutting properties when used for machining nodular cast iron. Nodular cast iron is a work piece material difficult to machine since it adheres onto the cutting edge of the tool resulting in a successive and fast removal of the coating from the cutting edge and, hence, a shortened tool life of the cutting inserts.
Al.sub.2 O.sub.3 crystallizes in several different phases: .alpha., .kappa., .gamma., .beta., .theta., etc. The two most frequently occurring phases in CVD of wear resistant Al.sub.2 O.sub.3 -coatings are the thermodynamically stable, hexagonal .alpha.-phase and the metastable .kappa.-phase. Generally, the .kappa.-phase is fine-grained with a grain size in the range 0.5-2.0 .mu.m and often exhibits a columnar coating morphology. Furthermore, .kappa.-Al.sub.2 O.sub.3 coatings are free from crystallographic defects and free from micropores or voids.
The .alpha.-Al.sub.2 O.sub.3 grains are usually coarser with a grain size of 1-6 .mu.m depending upon the deposition conditions. Porosity and crystallographic defects are in this case more common.
Often, both .alpha.- and .kappa.-phase are present in a CVD alumina coating deposited onto a cutting tool. In commercial cutting tools, Al.sub.2 O.sub.3 is always applied on TiC coated carbide or ceramic substrates (see, e.g., U.S. Pat. No. 3,837,896, now U.S. Reissue Pat. No. 29,420) and therefore the interfacial chemical reactions between the TiC surface and the alumina coating are of particular importance. In this context, the TiC layer should also be understood to include layers having the formula TiC.sub.x N.sub.y O.sub.z in which the carbon in TiC is completely or partly substituted by oxygen and/or nitrogen.
The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well-known as is evidenced in U.S. Reissue Pat. No. 29,420, and U.S. Pat. Nos. 4,399,168, 4,018,631, 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments, e.g., of TiC coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Although the disclosed methods result in alumina layers tightly and adherently bonded to the cemented carbide body or to a refractory layer of, e.g., TiC adjacent to the cemented carbide, they do not result in the particular .alpha.-polymorph of Al.sub.2 O.sub.3 as disclosed in the present invention.
Alumina coated bodies are further disclosed in U.S. Pat. No. 3,736,107, U.S. Ser. No. 08/128,741 and U.S. Pat. Nos. 5,137,774 and 5,162,147 wherein the Al.sub.2 O.sub.3 -layers comprise .alpha., .kappa. resp., .alpha./.kappa. combinations. However, these patents do not disclose the desired microstructure and crystallographic texture of the .alpha.-polymorph which is the object of the present invention.